There are many conventional methods for coating the outside surface of a cylindrical body with paint. Conventionally, where the smoothness of a coating film is of particular importance, a dip coating method has been used.
Generally, dip coating methods are employed to apply paint to a cylindrical body by dipping the cylindrical body into the paint and pulling the cylindrical body out of the paint at a particular speed The problem however, is that this method often times does not properly localize the paint on the outer surface of the cylindrical body. Rather, if the cylindrical body is opened at the upper and lower ends paint oftentimes enters the interior of the cylindrical body, thus coating paint not only on the outer surface but also the inner surface of the cylindrical body. Because the inner surface of a cylindrical body is coated with paint, paint is wasted and the inner surface of the cylindrical body does not properly dry and thus has the propensity to become contaminated.
In an effort to solve the aforementioned problems, a method has been suggested wherein a cover is attached to a lower portion of a cylindrical body to prevent paint from adhering to the inner surface of the cylindrical body. Specifically, Japanese Patent Unexamined Publication No. 59-142869 employs this suggested method.
Another conventional method, disclosed in Japanese Patent Unexamined Publication No. 58-186472, requires closing an upper portion of the cylindrical body. The problem with this method is that it necessitates bringing a thin inner pressure-detecting tube into the cylindrical body. Moreover, the method presents a problem because it requires additional devices such as a pressure sensor, a solenoid valve, and an electric circuit for controlling the pressure sensor and the solenoid valve.
In the coating method disclosed in Japanese Patent Unexamined Publication Nos. 59-4467 and 60-132678, a valve acting as the pressure adjustment mechanism is attached to the upper portion blocking device. When the cylindrical body is pulled out of the paint, the valve is immediately opened to adjust the air pressure in the cylindrical body prior to the liquid surface of the paint that has entered the cylindrical body reaches the lower end of the cylindrical body. This method is difficult because it involves precisely timing the adjustment for opening the valve at the point where the cylindrical body is pulled out of the paint. Moreover, this method cannot be applied to a cylindrical body which includes a closed upper portion and thus its use is limited to certain types of cylindrical bodies.
The pressure in a cylindrical body changes depending on the kind of solvent and the specific weather conditions it is subjected to, such as temperature and humidity. Accordingly, in the methods disclosed in the aforementioned Japanese Patent Unexamined Publications, it is difficult to achieve a pressure adjustment capable of modifying the pressure-adjustment conditions in accordance with various coating conditions. Further, because heretofore pressure-adjustment mechanisms have required a large number of moveable parts, dip coating apparatus has been complex and frequently has required excessive maintenance work.
In an effort to solve the problems outlined above, a method has been disclosed, for example-, in Japanese Patent Application No. 84743/61, wherein a space portion is formed in an upper portion of the cylindrical body to moderate the rising air pressure due to solvent evaporation. Typically, the space portion is closed but communicates with the inside of a cylindrical body. FIG. 5 shows the prior art method wherein a hollow cylindrical body 1 is held by a pillar supporting member 4, a grasping member 3, an 0-ring 5, and a cylindrical supporting member 2. A space portion S is formed in the cylindrical support member 2 and located at the top portion of the hollow cylindrical body for carrying out a pressure moderation function. The reference numerals 6 and 7 designate a coating tank and paint respectively. The difficulty with this method is it cannot be applied to a cylindrical body having a closed upper portion. Further, another problem with this method is that the supporting members are complicated in construction.